General Questions about Electricity

[Alan]

Should you be using language like that on the forum?

Apologies if emf means something other than electromagnetic field or electromotive force. I've only lived in the US for 27 years, I still haven't figured out the language differences

 

[NW-Bound]

But, why does the electron move at all since the current is reversing direction 60 times a second?

In an AC circuit, the electrons wiggle or boogie in place. They have no net motion, but their wiggling speed is still very slow.

If they move 14 in/hr in a 5A DC current, then with a 5A AC current, their peak velocity is still 14 in/hr. And if they have to reverse direction at 60 times/sec, they do not get very far very fast.

But let's go back to just the simple DC current for now, to consider the explanation to our layman how his lamp turns on immediately.

 

[NW-Bound]

Some of those cheap chargers could be fire hazards. I would be very careful if they did not come with the original equipment.

I have bought a couple of USB chargers to use with our iPhones off eBay. I paid $1.99 for a kit containing 1 wall charger, 1 cigarette plug charger, and 1 USB cable. Shipping from China included.

When I received it, carefully measured the output voltage to make sure it was 5V to avoid blowing up the phone. It looked OK. I have used them for a few months. They look just like the after-market chargers I have seen for a lot more money, those white 1" cubes you plug into a wall outlet.

 

[Fermion]

His lamp turns on instantly because the electron's electric field propagates through the wire at a significant fraction of the speed of light. It causes the electrons in the filament in the lightbulb to instantly move in response.

It is similar to what happens on the water when a boat passes by 100 feet away from your boat. If you were to dye a section of the water near the boat and watch it, you would see that the water actually doesn't move very far and yet a few seconds later the water near you moves and your boat moves. Since it takes energy to move your boat, that energy has been transmitted by the wave moving in the water, just like the energy is transmitted by the electric field moving in the wire. The water molecules are like the electrons. They both just slosh around a bit and don't really move very far very fast.

 

[scrinch]

... in a copper wire of cross-section 0.5 mm2, carrying a current of 5 A, the drift velocity of the electrons is on the order of a millimetre per second...

That's 3.6 m/hr or 14 in/hr.

Talk about inflation! Back when I was a kid, 3.6m/hr equaled 142 in/hr!

 

[Alan]

But let's go back to just the simple DC current for now, to consider the explanation to our layman how his lamp turns on immediately.

Because the charged particles (electrons) generate an emf that travels down the wire at the speed of light. For a DC circuit the direction of the magnetic field around a wire can be determined using Maxwells right hand corkscrew rule. (the magnetic and electric fields are always at right angles to one another)

The current that travels down a wire is the emf being passed between electrons. (I'll try and avoid terms like excited electrons in case REW reports me for using sexual innuendo). That is the layman's simple explanation, but I was never a teacher, so I'm sure someone like Professor Cox would do a much better job.

For a full explanation you would need to study Maxwell's equations.

 

[Alan]

His lamp turns on instantly because the electron's electric field propagates through the wire at a significant fraction of the speed of light. It causes the electrons in the filament in the lightbulb to instantly move in response.

It is similar to what happens on the water when a boat passes by 100 feet away from your boat. If you were to dye a section of the water near the boat and watch it, you would see that the water actually doesn't move very far and yet a few seconds later the water near you moves and your boat moves. Since it takes energy to move your boat, that energy has been transmitted by the wave moving in the water, just like the energy is transmitted by the electric field moving in the wire. The water molecules are like the electrons. They both just slosh around a bit and don't really move very far very fast.

Perfect

 

[NW-Bound]

+1

Here's another analogy that I would use.

Imagine hooking a long garden hose to a faucet, and that the garden hose is filled with water. The moment you open the faucet, water starts flowing immediately out the other end. The water molecules flowing out of the hose are not the same as the water molecules entering from the faucet.

...electric field propagates through the wire at a significant fraction of the speed of light...

The electron motion down the wire is started by the electric field propagation in the dielectric material surrounding the wire. The speed of propagation depends on the dielectric constant of the material and is usually a large fraction of the speed of light in vacuum as you noted.

 

[imoldernu]

Some of those cheap chargers could be fire hazards. I would be very careful if they did not come with the original equipment.

I get them at resale shops... most branded for whatever device they were designed for... toys, blackberry's, intercoms and all kinds of phones... Just counted my "supply"... actually over 100 chargers/power supplies... am the go to guy in the neighborhood when someone needs an extra plug in or car lighter adapter or laptop and printer cords.

One of my hobbies buying old electronics of all kinds... just to see how they work... six big plastic tubs full. Never pay more than $2.... My toys. Vic20, Adam, TRS80, Apple IIe, Sinclair, Emachine(s) etc...

 

[NW-Bound]

Talk about inflation! Back when I was a kid, 3.6m/hr equaled 142 in/hr!

Gotta be more careful with decimal points.

 

[Fermion]

The electron motion down the wire is started by the electric field propagation in the dielectric material surrounding the wire. The speed of propagation depends on the dielectric constant of the material and is usually a large fraction of the speed of light in vacuum as you noted.

Right, but you said explain to a layman so I wasn't going to get into the fact that the electric field was actually propagating in the air between the wires (for a common lamp cord).

I see you are just trying to keep it real.

 

[Lsbcal]

I get them at resale shops... most branded for whatever device they were designed for... toys, blackberry's, intercoms and all kinds of phones... Just counted my "supply"... actually over 100 chargers/power supplies... am the go to guy in the neighborhood when someone needs an extra plug in or car lighter adapter or laptop and printer cords.

One of my hobbies buying old electronics of all kinds... just to see how they work... six big plastic tubs full. Never pay more than $2.... My toys. Vic20, Adam, TRS80, Apple IIe, Sinclair, Emachine(s) etc...

I cannot honestly say that some chargers are dangerous but have read a bit about unfortunate events.

Whenever I charge something I try to make sure the charger and electronics are in an area that has very few flammable materials around. Like the kitchen tile area for instance. Maybe I'm too cautious but it makes me feel better.

 

[Fermion]

Harder to explain to a layman is an electric field propagating in a vacuum. Actually that is hard to explain to a EE student.

 

[imoldernu]

Re: electron flow... I had it explained to me, that the electrons don't actually move on a wire, but rather are absorbed by the atoms that constitute the transmission medium (in this case wire)... and thereafter being thrown off to the next atom, and so on down the line. That's an old, old memory gleaned from my college roomate, who had as his major thesis, the movement of electron (energy) in different gasses... shock tubes. 60 years ago...
I bow to newer technology...

 

[Lsbcal]

Many moons ago I had EE courses in Solid State Theory, Quantum Mechanics, E&M, etc. I never quite made the leap between the electron drift behavior in a wire and the EM field propagation. I'm aware of some of the loose qualitative explanations above so am not totally clueless.

Any of you have a good suggestion for a source that presents a qualitative explanation in a somewhat higher level discussion? If it had some equations I could perhaps handle that ... maybe.

I'm sure the OP did not expect us to get sidetracked in the land of physics. Sorry about that.

 

[ERD50]

Here is a little tidbit. Did you know that electrons in a wire only move about 3 inches an hour? Talk about slow!

Right, it is a wave (but I actually was unaware of those numbers - interesting).

But it it so much more 'fun' to imagine our little electrons zipping through the wires in their little hot rods, leaning into the tight turns!

But like this little desk toy, the center balls barely move, but the wave propagates very quickly.

ooops, I missed a bunch of posts while I was writing this and doing other things! Oh well, it might still fit.


-ERD50

 

[Fermion]

To understand better EM propagation, especially in a vacuum (where there is no obvious medium like the water in the hose example), try Richard Feynman's book, "QED, The Strange Theory of Matter and Light".

I haven't read it in a long time. I should reread it. It is like $11 on Amazon.

 

[NW-Bound]

Re: electron flow... I had it explained to me, that the electrons don't actually move on a wire, but rather are absorbed by the atoms that constitute the transmission medium (in this case wire)... and thereafter being thrown off to the next atom, and so on down the line...

Yes. Each copper atom has one free electron (not all electrons can escape from their orbit), and this free electron can jump from one atom to the next.

By the way, in contrast with electron speed in a wire, free electrons inside a CRT of the old TVs do actually move very fast in a beam. I remember speed numbers like 1/4 the speed of light.

Knowing that electrons in a TV CRT are accelerated by a potential of 20+KV, I can sit down and verify the above speed number, but will not. I have problems with placing decimal points, as you have seen.

 

[target2019]

When you're looking at costs and savings, the next kWh you save is more expensive than the average. Say my average is a dime, I use 15 cents when I calculate the cost of running an additional fan, for instance.

 

[ERD50]

True.

But how do you explain in simple terms to a layman ....

shhhhhh! You don't explain it in simple terms to a layman! Don't you remember the oath we all took? How are we going to make the big bucks and seem 'special' (yes, I'm throwing an easy one across the plate, trying to make it too easy to be worth an 'attack'!) if everybody understands this stuff?

One more slip, and you will be sent back for 'reprogramming'.

Harder to explain to a layman is an electric field propagating in a vacuum. Actually that is hard to explain to a EE student.

Or even someone who hasn't thought much about that since he was a student, a long time ago.

-ERD50

 

[ERD50]

When you're looking at costs and savings, the next kWh you save is more expensive than the average. Say my average is a dime, I use 15 cents when I calculate the cost of running an additional fan, for instance.

Depends. If you are not on tiered rates, that's not the case. And if you have fixed fees and are on a single rate, the marginal rates are actually lower than average, but it's rather meaningless if you can't do anything about the fixed rate portion. But mathematically, that first kWh is expensive.

-ERD50

 

[M Paquette]

Reduction to practice is an exercise left for the reader.

 

[NW-Bound]

Just mention electrons and a thread starts drawing in nerds like you couldn't believe...

 

[Lsbcal]

To understand better EM propagation, especially in a vacuum (where there is no obvious medium like the water in the hose example), try Richard Feynman's book, "QED, The Strange Theory of Matter and Light".

I haven't read it in a long time. I should reread it. It is like $11 on Amazon.

I have it but got distracted. Probably by NW-bound in some stock market threads.

Guess I don't have any excuses now. Thanks for the pointer.

 

[NW-Bound]

I had a few books by Feynman, but not that one.

Arghh... Why would I want to get back in something like this? I begin to wonder if I will ever build anything again, or write any more software or firmware. Two scopes, 4 signal generators, two spectrum analyzers, only one has been turned on recently.

I like money too much, and counting it is easier. Actually Quicken does all the counting, as I do not even have to add up the numbers. I am getting soft, really soft...